The development of malignancy and of drug resistance are two prominent characteristics of the evolution of neoplastic cell populations, Both of these properties result from genomic instability and the cellular heterogeneity which is produced. To date, the best studied example of genomic instability and cellular heterogeneity is amplification of dihydrofolate reductase (dhfr) gene as a cellular response to the chemotherapeutic drug, methotrexate. The longterm objective of this research project is to understand the role of genomic instability as exemplified by dhfr gene amplification. The underlying hypotheses for these studies is that dhfr gene amplification is the result of a general cellular process that is induced by environmental stress. This pleiotropic response generates heterogeneity in the cellular population and may be instrumental in the generation of tumors. We will ask if the enhancement of the incidence of dhfr amplification (by UV radiation) can be transmitted to other cells which have not been irradiated. We will accomplish this aim by using cell fusion studies as well as coincubation studies. Enhancements of dhfr gene amplification in nonirradiated cells through coincubation or fusion with irradiated cells would indicated a regulatory role for UVinduced products in the amplification event which may be transmitted to nonirradiated cells (i.e., transacting factors). We will investigate the expression of genomic instability in transformed an nontransformed cells using the incidence of dhfr amplification as a marker. If transformed cells exhibit an increase in genomic instability (increased incidence of dhfr gene amplification) we will ask if it can be transmitted to nontransformed cells using the coincubation and fusion studies used previously. Finally, we will examine the relationship between increased binding of fluoresceinated MTX after irradiation and the production of dhfr protein.